Endocardial pacing leads having fixation structures at or near the distal tip end are well known in the art. A widely used anchoring arrangement is that of the tined lead, where a plurality of pliant tines are disposed near the distal tip to effect good chronic anchoring after the lead tip has been optimally positioned in the heart. While the design has had some problems, the tined anchor has become the most popular passive fixation mechanism for endocardial pacemaker leads. The patent literature contains many disclosures of tined pacing leads. See, for example, U.S. Pat. Nos. 3,902,501; 3,939,843; 4,033,357; 4,236,529; 4,301,815; 4,409,994; and 5,582,069. Additionally, the non-patent pacing literature has disclosed and discussed the advantages of the tine-type anchor for chronic fixation, and the acceptance of this type of fixation mechanism in the pacing industry.
The fixation mechanism provided by tines is well understood. When heart wall growth, or trabeculae, become lodged within the acute angle formed between the tines and the tube casing, this provides a substantial anchoring effect. The tines must be designed so that they can fold down over the tubing, to reduce the introducer width at time of implanting the lead, and must be able to bend forward (distally) toward the distal tip if it is necessary to dislodge the lead during the acute stage, particularly during initial placement of the lead. This means that the tine characteristics must be chosen carefully to permit easy folding of the tine down onto the casing, and also to provide the right amount of resistance to forward bending of the tine as may be necessary during the acute stage of lead placement. It has been found that one problem that occurs during placement of the tined lead is that the tip becomes entangled in chordae connected to the heart valve, necessitating relative withdrawal of the lead and thus forward bending of the tines. Also, an obvious disadvantage of the tined structure is that when folded back upon the lead during introduction, the tines add to the thickness of the lead, i.e. increasing the introducer size.
Early tine lead designs had relatively large introducer sizes. With the venous introducer technique becoming more popular, tined lead designs were improved so that the lead would fit into smaller introducers. The introducer size of the lead is the minimum tube diameter through which a lead can be pushed with a minimum amount of friction. In practice, it has been determined primarily by: (a) the diameters of the rigid parts in the lead tip and body; (b) the elasticity of the deformable parts of the lead tip and body, since these determine friction forces; and (c) the coefficient of friction between the lead and the introducer, which can be affected by the choice of low friction materials. A classical tined anchor design is a trade-off between rigidity for good anchoring, or holding force, and flexibility for acute removability, e.g., from the chordae of papillary muscles or other unintended positions. The tine bending rigidity is substantially the same in each direction. This rigidity is an advantage when bending the tine forward, since it provides a good holding function, but it is a disadvantage when bending the tine backwards during introduction of the lead.
FIGS. 1A and 1B of this application present a representation of a prior art tined silicone lead design which is representative of the design compromises of the prior art. As can be seen in FIG. 1A, and as is conventional with tined lead construction, the normal position of the tine is to slope backward proximally from the distal tip, at an acute angle with respect to the lead casing. However, as seen in FIG. 1B, during introduction the tines must be folded backward (proximally) upon the casing, causing a substantial increase in the overall width at that point, limiting the introducer size.
Another typical problem with the prior art tined lead design is that the lead casing, either silicone tubing or PUR tubing, is not extended all the way forward to the tip. In the design of FIGS. 1A and 1B, the distal end of the coil is attached to the tip element by crimping, the crimping sleeve necessarily requiring a greater radius than that of the coil. The tined anchor piece is placed over the crimping sleeve and the silicone tubing, causing a bulge at the point of overlap with the silicone tubing. Note that this overlap is necessary in order to provide good sealing, and to prevent flow of body fluids into the coil area. Thus, the conventional crimp arrangement provides an additional limitation to reduction of the introducer size.
In practice, the tined lead as exemplified by FIG. 1A and 1B is introduced with the use of a stylet, the distal end of which presses up against the inner core piece of the tip. A problem that exists in practice is that the introduction of the stylet and the maneuvering of the lead and stylet into the heart can result in the distal end of the stylet poking through the coil near the distal tip end, which can be ruinous to the lead. It is thus advantageous to include, in any lead design, means for preventing the piercing of the distal lead end by the stylet while it is being used. The crimping sleeve solves this problem, but at the expense of introducer size.
Another design feature of importance is the inner radius R (as seen in FIG. 1A) between the tine and the longitudinal surface of the lead. In practice, this radius is a compromise. A small radius gives the best flexibility to the tine, i.e., reduces its stiffness to bending in either direction. If the design anticipates the placement of the lead will involve undesired catching of heart wall structures, e g. chordae, a larger radius is desired to better permit the heart structure to bend the tine forward, for extraction of the lead. However, a larger inner radius R will negatively affect the bending capability of the tine, and will increase the introducer size
The tined lead of FIGS. 1A and 1B, made commercially by the assignee of this application, fits into a 13 F introducer with its tines folded backward Further tine flattening permits reduction to an 11 F size. Since the body size is 5.5 F, the introducer size is seen to be twice as great. By contrast, the objective of the lead of this design is an 8F introducer size for a silicone lead, and a 6F size for a lead made with PUR 55D.
There thus have existed a number of problems associated with lead tip design, which the design of the instant invention reduces. The problem of introducer thickness is achieved by an improvement in the cross-sectional design of the tine and by extending the lead tubing all the way to the forward portion of the tip element. Instead of crimping the end electrode piece to the coil, the inside of the electrode is spot welded to the distal end of the coil, enabling extending the lead tubing all the way to the tip end. The problem of optimally selecting the inner radius R is alleviated by the introduction of riser elements between respective pairs of tines. Further, the problem of ensuring against piercing the lead during use of the stylet is reduced by providing a sheathing around the coil for a distance proximal to where the coil is attached to the tip element.